Final answer:
The enthalpy change for the combustion of a substance is calculated using stoichiometry, which relates the moles of reactant to the energy change. For instance, combustion of methane releases 890.4 kJ/mol, so doubling the amount doubles the energy release. This method applies to any combustion reaction, allowing calculation of enthalpy changes for any specified amount of reactants.
Step-by-step explanation:
When calculating the enthalpy change of the combustion of a substance, we use stoichiometry to relate moles of reactants to the energy released or absorbed. As an example with methane, the combustion of 1 mole releases 890.4 kJ of energy. Therefore, combusting 2 moles would release double the energy, which is 1781 kJ, and 0.5 moles would release half, which is 445.2 kJ.
To calculate the enthalpy of combustion for a different substance, like ethanol, C₂H₅OH, we start by writing a balanced equation for its combustion to CO₂(g) and H₂O(g). We then use enthalpy values from a reference (like Appendix G) to calculate the heat released per mole of ethanol combusted. For each substance, the enthalpy change for the reaction is given as a change in kJ per mole of the substance reacted.
Applying Stoichiometric Calculations
Stoichiometric calculations allow us to relate the amount of a substance, in moles, to the corresponding enthalpy change. For example, if we know the enthalpy of combustion of sulfur dioxide (SO₂), we can calculate the energy change for any given number of moles. Similarly, if we are given the enthalpy change per mole for a substance, like 744 kJ/mol KClO₃, we can also calculate the total energy change for any stoichiometric amount of it by multiplying the per-mole energy by the number of moles.